There is a lot of information on the sidewall of a tire. Typically, you'll find UTQG ratings for treadwear, traction and temperature, the size of the tire, the load rating index number with a speed rating index, the construction type (bias or radial), the D.O.T. compliance code, construction details, and of course, the make and model of the tire. On some tires used as original equipment, you may also find a marking that indicates its OE status. Porsche uses an N-0 or N-1 designation, BMW uses a star on some O.E. tires and General Motors uses a "TPC" code. Light Truck tires are sometimes marked with an LT for "Light Truck" before the size, passenger tires are often marked with the letter P for "Passenger" before the size. Passenger tires of the same size with or without the P are virtually interchangeable.

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Air Pressure:
Your tires support the weight of your vehicle, right? Well, they don’t! It’s the air pressure inside them that actually supports the weight. Maintaining sufficient air pressure is required if your tires are to provide all of the handling, traction and durability of which they are capable.

However, you can't set tire pressure...and then forget about it! Tire pressure has to be checked periodically to assure that the influences of time, changes in ambient temperatures or that a small tread puncture has caused it to change.

The tire pressure recommended in your vehicle's owner's manual or tire information placard is the vehicle’s recommended "cold" tire inflation pressure. This means that it should be checked in the morning before you drive more than a few miles, or before rising ambient temperatures or the sun’s radiant heat affects it.

Since air is a gas, it expands when heated and contracts when cooled. In most parts of North America, this makes fall and early winter months the most critical times to check inflation pressures...days are getting shorter...ambient temperatures are getting colder...and your tires' inflation pressure is going down!

The rule of thumb is for every 10° Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi (up with higher temperatures and down with lower).

In most parts of North America, the difference between average summer and winter temperatures is about -50° Fahrenheit...which results in a potential "loss" of about 5 psi as winter’s temperatures set in. And a 5 psi "loss" is enough to sacrifice handling, traction, and durability!

Additionally, the difference between cold nighttime temperatures and hot daytime temperatures in most parts of the country is about 20° Fahrenheit. This means that after setting tire pressures first thing in the morning, the vehicle’s tire pressures will be almost 2 psi higher when measured in the afternoon (if the vehicle was parked in the shade). While that is expected, the problem is when you set your vehicle’s tire pressures in the heat of the day, their cold pressures will probably be 2 psi low the following morning.

And finally, if the vehicle is parked in the sun, the sun’s radiant heat will artificially and temporarily increase tire pressures.

We even evaluated the effects of heat generated by the tire’s flexing during use. We tried to eliminate the variable conditions we might encounter on the road by conducting this test using our "competition tire heat cycling service" that rolls the tires under load against the machine’s rollers to simulate real world driving. We monitored the changes in tire pressure in 5-minute intervals. The test tires were inflated to 15 psi, 20 psi, 25 psi and 30 psi. Running them all under the same load, the air pressure in all of the tires went up about 1 psi during every 5 minutes of use for the first 20 minutes of operation. Then the air pressures stabilized, typically gaining no more than 1 psi of additional pressure during the next 20 minutes. This means that even a short drive to inflate your tires will result in tires that will probably be "underinflated" by a few psi the following morning.

Add all of these together, and you can understand why the conditions in which you set your vehicle’s tire pressures are almost as important as the fact that you do set it.

It’s important to remember that your vehicle's recommended tire pressure is its "cold" tire inflation pressure. It should be checked in the morning before you drive more than a few miles, or rising ambient temperatures or sun’s radiant heat affects it.

And by the way, if you live in the North and park in an attached or heated garage you will "lose" pressure when you leave its warmth and venture into the real world outside during winter. Add 1 psi "cold" pressure tire pressure to compensate for each 10° Fahrenheit temperature difference between the temperature in the garage and outside.

Checking your air Pressure:
When vehicle manufacturers select a tire size for a vehicle, they evaluate the vehicle’s gross axle weights, the anticipated use of the tire, and the tire diameter and width. Adjustments to these factors give the manufacturer a way to improve handling and appearance. This is especially true for performance tire sizes. The size selected is rarely limited to only one capability (i.e. carrying the vehicle’s weight). The tire usually needs to have additional load capacity as well. This extra capacity is important because without it all of the tire’s performance would be used up just carrying the weight of the vehicle and little would be left for durability at high speeds or responsive handling. For all vehicles produced since 1968, the original tires sizes and inflation pressures (including the spare) are listed on a vehicle placard. This placard can be located on:

The driver-side door or door jamb (Ford vehicles on the rear passenger door jamb)
Glove box or counsel door
Fuel filler door
The engine compartment
Additionally, some manufacturers also list the original tire pressure in the vehicle’s owner’s manual.If a car’s inflation pressure has varied from that which was recommended by the manufacturer, it’s likely that the tire’s wear and performance characteristics have also changed.

Underinflation

If your vehicle’s tires are underinflated by only 6 psi it could lead to tire failure. Additionally, the tire’s tread life could be reduced by as much as 25%. Lower inflation pressure will allow the tire to deflect (bend) more as it rolls. This will build up internal heat, increase rolling resistance and cause a reduction in fuel economy of up to 5%. You would find a significant loss of steering precision and cornering stability. While 6 psi doesn’t seem excessively low, remember, it usually represents about 20% of the tire’s recommended pressure.

Overinflation

If your tires are overinflated by 6 psi, they could be damaged more easily when running over potholes or debris in the road. Higher inflated tires cannot isolate road irregularities well causing them to ride harsher. However, higher inflation pressures usually provide an improvement in steering response and cornering stability up to a point. This is why participants who use street tires in autocrosses, track events and road races, run higher than normal inflation pressures.

The pressure must be checked with a quality air gauge as the inflation pressure can not be accurately estimated through visual inspection.
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Tire Dimensions:
The first number is the width of the tire in millimeters, measured from sidewall to sidewall. To convert to inches, divide by 25.4 In the example above, the width is 185mm or 7.28".

The second number is the aspect ratio. This is a ratio of sidewall height to width. In the example above, the tire is 7.28" wide, multiply that by the aspect ratio to find the height of one sidewall. In this case, 185x0.60=111mm or 7.28"x0.60=4.36".

The last number is the diameter of the wheel in inches.

To figure the outside diameter of a tire, take the sidewall height and multiply by 2 (remember that the diameter is made up of 2 sidewalls, the one above the wheel, and the one below the wheel) and add the diameter of the wheel to get your answer.

Example...185/60R14 85H or 185/60HR14

185mm x .60=111mm x 2=222mm + 355.6mm(14")= 577.6mm or 22.74"

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Speed Ratings:
It is important to note that speed ratings only apply to tires that have not been damaged, altered, underinflated or overloaded. Additionally, most tire manufacturers maintain that a tire that has been cut or punctured no longer retains the tire manufacturer’s original speed rating, even after being repaired.

In Europe, where selected highways do not have speed limits and high speed driving is permitted, speed ratings were established to match the speed capability of tires with the top speed capabilities of the vehicles to which they are applied. Speed ratings are established in kilometers per hour and subsequently converted to miles per hour (which explains why speed ratings appear established at “odd” mile per hour increments). Despite the tire manufacturer’s ability to manufacturer tires capable of high speeds, none of them recommend the use of their products in excess of legal speed limits.

Speed ratings are based on laboratory tests where the tire is pressed (to reflect its required load) against a large diameter metal drum and run at ever increasing speeds (in 6.2 mph steps in 10 minute increments) until the tire’s required speed has been met.

*Today, the Z-speed rating is the only speed rating that is still branded “within” the tire size, as in P225/50ZR16. All other speed ratings are shown in the tire’s service description.

When Z-speed rated tires were first introduced, they were thought to reflect the highest speed rating that would ever be required. Since that time the automotive industry has found it necessary to add W- and Y-speed ratings (indicated in the tire’s service description) to identify the tires that meet the needs of new vehicles that have extremely high, top speed capabilities.

While all Z-speed rated tires are capable of speeds of 149 mph and above, prior to the W- and Y-speed ratings were identified in the service, how far above 149 mph was not identified.

Prior to 1991, the most popular speed ratings were “S,” “H” and “V.” However, while the speed capabilities of S- and H-rated tires still indicate the same speeds as before, the V-speed rating has been modified. Previously a V-speed rated tire with the “V” branded “within” the tire size indicated that the tire was capable of 130+ miles per hour. Today’s new V-speed rated tires are always identified in the tires service description

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Tire Rotation:
Tire rotation can be beneficial in several ways. When done at the recommended times, it can preserve balanced handling and traction of the tires and even out tire wear. It can even provide performance advantages. When should tires be rotated ? We recommend that high performance tires be rotated every 3,000 to 5,000 miles, even if they don't show signs of wear. Tire rotation can often be done with oil change intervals while the vehicle is off the ground anyway. Tire rotation helps even out tire wear by allowing each tire to serve in as many of the vehicle's wheel positions as possible. Remember, tire rotation can't correct wear problems due to worn mechanical parts or incorrect inflation pressures. It's also important to check your owner's manual for specific details on what method of tire rotation the vehicle's manufacturer recommends.

While every vehicle is equipped with four tires, usually the tires on the front need to accomplish very different tasks than the rear tires. And the tasks encountered on a front wheel drive car are considerably different than those of a rear wheel drive car. Tire wear experienced on a performance vehicle will usually be more severe than those on a family sedan. Each wheel position can cause different wear rates and different type of tire wear.

While no one likes their tires to wear out, it is actually an advantage when all of the tires on a vehicle wear at the same rate throughout their life. As tire wear reduces tread depth, it allows the tires to respond to the driver's input more quickly and increases dry road performance. Since tire rotation will help all of the vehicle’s tires wear at the same rate, it will keep the tires performing equally on all four corners.

When your tires wear out together you can get a new set of tires, without being forced to buy pairs. If you replace tires in sets you will maintain the original handling balance. And our suppliers are constantly introducing new tires, each of which improves upon their past product's performance. If you replace your tires in sets, it allows you to experience today's technology, instead of being forced to match yesterdays.

On front wheel drive cars, rotate the tires in a forward cross pattern.
On rear wheel or four wheel drive vehicles, rotate the tires in a rearward cross pattern.
If you car has directional wheels or tires, rotate them front to rear.
If you car has non-directional tires that are a different size from front to rear, rotate them across the axle.

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Refer to 'Miles/FocusFanatics' as your previous contact when you order online. You can also credit the forum by ordering through any [URL="http://www.tirerack.com/?affiliate=AD4"]Tire Rack[/URL] link.

Performance benefits:
While many people choose alloy wheels for their beauty, there are equally important performance benefits to be derived including...

Reduced Unsprung Weight Compared to Steel Wheels
This is one of the most critical factors affecting a vehicle's road holding ability. Unsprung weight is that portion of a vehicle that is not supported by the suspension (i.e. wheels, tires and brakes) and therefore most susceptible to road shock and cornering forces. By reducing unsprung weight, alloy wheels provide more precise steering input and improved "turning in" characteristics.

Improved Acceleration and Braking
By reducing the weight of the vehicle's rotational mass, alloy wheels provide more responsive acceleration and braking.

Added Rigidity
The added strength of a quality alloy wheel can significantly reduce wheel/tire deflection in cornering. This is particularly critical with an automobile equipped with high performance tires where lateral forces may approach 1.0g.

Increased Brake Cooling
The metals in alloy wheels are excellent conductors of heat - improving heat dissipation from the brakes - reducing risk of brake fade under demanding conditions. Additionally, alloy wheels can be designed to allow more cooling air to flow over the brakes.
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What determines quality?
The Tire Rack’s quality standard for wheels is very high and the manufacturers that we represent in this market understand that we constantly monitor products to ensure that quality products are sold to our customers. But what determines quality?

Manufacturing Process
Manufacturing processes and levels of testing are critical to a wheel's structural integrity. (Read more in "Wheel Construction.") International quality standards such as ISO9001, QS9000, TUV of Germany or VIA of Japan, establish important production and quality standards that manufacturers must follow. In addition, dimensional tolerances based on strict, original equipment market standards versus the more “casual” standards allowed for many aftermarket products should be met. Even durability standards for finish are different between the original equipment market and the aftermarket.

Proper Fit
An accurate fitment is the difference between good, better and best. Critical wheel dimensions such as width, diameter, offset, center bore, brake clearance, as well as load factor and lug hardware are the basics when it comes to properly fitting aftermarket wheels. Installation also requires a high level of sophistication. Many new vehicles are available with features such as ABS, traction control and other features that create a more difficult environment for installing aftermarket wheels. Stability control systems, run-flat tires, large high performance brake systems and staggered wheel and tire sizes are also factors to be considered when establishing accurate fitments. Wheel manufacturers with product design, research and development teams work to determine proper fitment as part of the manufacturing process.

Protective Finish
The type and quality of protective finish on your wheel (as well as proper maintenance) will determine how your wheels look years from now. Check for finish warranties backed by manufacturers with outstanding reputations for quality.

Reputation and Heritage
The reputation of a manufacturer is a strong indicator of quality since it is quality upon which a distinguished reputation is built. It takes time to build a positive reputation and a commitment to maintain it is important. And know a wheel company's roots. Many wheel manufacturers first established themselves in the motorsports arena and apply that technological and philosophical foundation to their production of wheels for use on the street.

The Tire Rack's Commitment to Quality
The Tire Rack constantly reviews wheel data from new vehicles to be sure that we are aware of the original equipment sizes and packages offered. We physically inspect many of today’s new vehicles and often supply technical data to some of the manufacturers outside the U.S. that may not have access to certain vehicles in our market. For many wheels that we import or represent, we specify certain dimensions that we require to ensure wheel fit and maintain our high quality standards.
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Offset:

The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types.

-Zero Offset
The hub mounting surface is even with the centerline of the wheel.

-Positive
The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.

-Negative
The hub mounting surface is toward the back or brake side of the wheels centerline. "Deep dish" wheels are typically a negative offset.

If the offset of the wheel is not correct for the car, the handling can be adversely affected. When the width of the wheel changes, the offset also changes numerically. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside. For most cars, this won't work correctly. We have test fitted thousands of different vehicles for proper fitment. Our extensive database allows our sales staff to offer you the perfect fit for your vehicle.
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Torquing lug hardware:
Proper installation requires that the wheel lug torque be set to the recommended specification for your vehicle. These torque specifications can be found in your vehicle’s shop manual or obtained from your vehicle dealer. Finish tightening the lugs down with an accurate torque wrench. Use a crisscross sequence (shown below) until they have reached their proper torque value. Be careful because if you over torque a wheel, you can strip a lug nut, stretch or break a wheel stud, and cause the wheel, brake rotor and/or brake drum to distort.

NOTE: When installing new wheels you should re-torque them after traveling the first 50 to 100 miles. This is necessary because as the wheels are “breaking in” they may compress slightly allowing their lugs to lose some of their torque. Simply repeat the same torque procedure listed above.

A direct tire pressure monitoring systems (TPMS) will be standard equipment on every vehicle sold in the U.S. after September, 2007. The criteria for TPMS standardization is that the vehicle must have only four wheels (no dual-wheeled trucks or motorcycles), and weigh less than 10,000 lbs.

The Focus has had TPMS since the 2008 models were released. Lear sensors are used and are in stock at the Tire Rack for all Fords up to the current model year. The size of the sensor shouldn’t be a problem for many wheels, but if the valve hole is in the middle of the wheel’s barrel, the sensor will not fit properly. Wheel manufacturers are aware of the issue and are rushing to get most of their wheels TPMS-compatible.

The Tire Rack's fitment specialists have carefully determined which wheels that are currently available will be compatible with the system installed on your vehicle.

: : : : : : : : : : : : : : : : : : : : : : : : : : : : : :

If you have any questions about the wheel you're considering, call or post your question for confirmation that it will be compatible with your TPMS.

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TPMS Facts

Direct TPMS use a sensor in the wheel/tire to wirelessly relay pressure back to the driver via a display inside the vehicle. Some vehicles have a digital screen that continuously shows all four tire pressures (five if spare has sensor). Others simply flash a "low tire pressure" light on the dash.

Additionally, there are two different styles of sensors for a direct system: a valve sensor and a strap/band sensor. The strap/band sensor is common place in most Fords. If you bought an aftermarket SmarTire TPMS, its sensors are held on by a strap around the barrel of the wheel like the Ford systems.

Indirect systems are also common on other vehicles and work in conjunction with the ABS wheel speed sensors that “count” the number of revolutions of each tire. If the right front tire is low (25psi) it will spin faster than the left front tire (35psi). The ABS system will recognize this change and alert the driver with a flashing light and possibly a loud chime. Indirect systems do not affect the fitment of aftermarket wheels.

__________________
Refer to 'Miles/FocusFanatics' as your previous contact when you order online. You can also credit the forum by ordering through any [URL="http://www.tirerack.com/?affiliate=AD4"]Tire Rack[/URL] link.

Looking at diagrams of wheel measurements, it seems that backspacing is the distance from the inner rim of the wheel to the hub mounting point. This can be calculated by dividing the wheel's width in half (thus determining the centerline) and then adding (or subtracting) the wheel's offset.

With many of the wheels said to fit a 2012 Focus (5x108 bolt pattern, high positive offset), it seems their measurements are incorrect. For example:

One or more of these measurements must be incorrect. An 8" wide wheel's centerline is 4", or 101.6 mm. Add the offset and you get 139.6 mm of backspacing, which is about 5.5". That's off by over half an inch.

These kinds of measurements are crucial on newer cars with ever-increasing factory offsets. Are these numbers supplied by the manufacturer?

EDIT: Did a bit more research and found that wheel width is not equal to overall wheel width, and 1" is a relatively safe amount to add to determine overall width. That means this particular wheel would have a centerline at approximately 4.5", and adding 38 mm of offset (1.496") would put you at 5.996" of backspacing. Looks to be more accurate, but to get 6.06" of backspacing this wheel's overall width would have to be 9.128".

Great information, thanks. How can I find out what the stock offset is on my 2006 Focus ZXW (steel wheels with hubcaps) so I can select the correct offset in replacement alloy wheels? I appreciate any suggestions!

I would just like to add a bit more information to this post. It was very well written and explains a lot in detail.

Underinflation: While everything posted was true, there are a few things that can be added. Underinflation can also cause shoulder wear on both sides of the tread, can cause belt seperations, and the innerliner of the tire to disintegrate and crumble due to heat build up. Which in the case of the innerliner disintegrating and you go to inflate the tire, you run about a 75% chance of the tire blowing up in your face. If the tire has been underinflated it is best to dismount the tire and inspect the innerliner. Also low inflation pressure can cause irregular wear which in most cases will void your warranty, cupping or heal and toe wear as it is called, which in turn causes noise is an effect of underinflation.

Overinflation: This can cause center wear in a tire and premature wear.

Speed ratings: Although this is primarily performance ratings on the tire for speed, it more or less boils down to braking distance and handling characteristics. You will never ever want to run two different speed rated tires on one vehicle. You will also never want to drop your speed rating as this will increase both your braking distance and the handling characteristics of the vehicle. You may increase the speed rating, but you will be sacrificing the comfort and quietness of the vehicle.

O.E. tires: These tires are spec'd out be the vehicle's manufacturer, most do not carry a tread mileage warranty as they are tires that are primarily designed for better fuel economy and not for longevity. The are not crappy or poorly built tires, however built with a totally different objective in mind and that is to get the best fuel mileage possible.

Defects: This has to be the most construed perception of a tire by customers. Blow-outs, bubbles that occur on the sides of tires, and loss of air are not manufacturing defects. They are all things that are caused by abuse or road hazards.

I am no expert on tires, however I do work with them every single day. I have a wealth of information and love to share. Any questions feel free to contact me as I am more than willing to help in any way I can.

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